1. Technical Field
The present invention relates to an optical substrate, a manufacturing method thereof, a planar lighting device and an electrooptical device.
2. Related Art
A liquid crystal display device, which is a kind of electrooptical device, typically has a backlight which is a planar lighting device illuminating a plane and provided on the back side of a liquid crystal panel. So called edge-light type backlights are widely used for the liquid crystal display devices since it is possible to make the device thinner with the edge type backlight. In the edge type backlight, light that entered around a side face of the liquid crystal panel is led to the back side of the liquid crystal panel (to the lighting device side) and illuminates the panel.
JP-A-2002-298626 is an example of related art. The edge-light type backlight typically has a light source such as a light emitting diode (LED) and a light guide plate in which the light emitted from the light source is internally reflected and led to the side face of the lighting device (an exit face). Furthermore, the backlight has a diffusing sheet provided on the light exit side of the light guide plate and a reflecting sheet provided on a side (a reflect face) which is the opposite side of the light exit side of the light guide plate in order to equalize and to increase the brightness of the light coming out from the light guide plate.
Light emitted from the light source enters into the light guide plate. The entered light is refracted and reflected between the exit face and the reflect face (or the reflecting sheet), then led to the substantially whole face of the exit face. Light led to the exit face enters the diffusing sheet, is diffused by optical elements (for example, microlenses) formed on the diffusing sheet, and then exits to the lighting device side. In this way, unevenness in the brightness, such as bright points and bright lines in the light guide plate is diffused, and light emitted from the light source can evenly illuminate the liquid crystal panel.
However, the above-described backlight has many members and components (for example, the above mentioned diffusing sheet, the light guide plate, the reflecting sheet and the like) in order to increase the brightness and to equalize the unevenness in the brightness of the illumination. This could be a problem because it reduces the productivity of the backlight. Such problem can be solved by providing the optical elements (for example, microlenses) formed on the diffusing sheet to the light guide plate. In other words, the diffusing sheet can be omitted by providing the light diffusing property directly on the light guide plate. In this way, the number of the members used for the back light can be decreased and it is possible to prevent the productivity of the backlight from being reduced.
Meanwhile, positional orientations of the members in the above-described backlight are decided based on the position of the light source so that it takes time to assemble and the productivity could be reduced. If a mark (for example, an alignment mark) showing an orientation to provide a member is formed on the light guide plate, the assembling process would become easier and the productivity of the backlight would not be reduced. For this reason, forming a part showing the layout orientation of the member such as the alignment mark on the light guide plate has been requested.
However, where it is attempted to form the alignment mark and the like on the light guide plate having the microlens, the microlens could be damaged during the formation process of the alignment mark. In addition, the cost and the number of steps in the process would be increased in order to form the alignment mark. This could reduce the productivity of the light guide plate and the productivity of the backlight could not be improved.